Adds Invalid_opcode of int exception and extension_dispatch_ref forward
ref (default raises Invalid_opcode op), plus the |op when op >= 200 arm
before the catch-all in the bytecode dispatch loop. Partition comment
documents 1-199 core / 200-247 extensions / 248-255 reserved.
Phase B will install the real registry's dispatch into the ref at module
init, replacing this stub.
Tests: 4 new foundation cases (Invalid_opcode for 200/224/247, Eval_error
for 199 to pin the threshold). +4 pass vs baseline, no regressions.
plans/sx-vm-opcode-extension.md ports over from loops/erlang (f6a68656)
with the opcode partition adjusted to match real VM usage: 1-199 core
(current ceiling 175 = OP_DEC), 200-247 extensions, 248-255 reserved.
plans/agent-briefings/sx-vm-extensions-loop.md captures the per-fire
workflow and ground rules.
